The first article that I used as a reference for Zeetah I’s solver was published in Byte Magazine back in 1987. Fortunately for me, this article was printed right around the time we were making Zeetah I.
Stephen E. Belter, Computer-aided Routing of Printed Circuit Boards: an Examination of Lee’s Algorithm and Possible Enhancements, BYTE, June 1987, pp. 199-208 http://dl.acm.org/citation.cfm?id=26020&CFID=154590879&CFTOKEN=60826616
Another article that does a good job of describing Lee’s algorithm can be found at: http://collaboration.cmc.ec.gc.ca/science/rpn/biblio/ddj/Website/articles/DDJ/1989/8909/8909a/8909a.htm